Britain has for the first time raised the spectre of al-Qaida operating in Syria, while at the same time accusing Damascus of brutally targeting specific communities and driving Syrians to take up arms.

The foreign secretary, William Hague, said regime forces were bombarding neighbourhoods then unleashing militia groups to murder civilians in their homes. He said more sanctions against the regime were likely if the UN-brokered peace plan continued to fail, and again appeared to leave open an option for some sort of intervention in the rapidly deteriorating situation in Syria.

Hague said security assessments had indicated the presence in Syria of al-Qaida, a group disavowed by the main opposition force, the Free Syria Army, but who regime officials insist are at the vanguard of a now raging insurgency.

“We … have reason to believe that terrorist groups affiliated to al-Qaida have committed attacks designed to exacerbate the violence, with serious implications for international security,” said Hague in a speech to the Commons.

He offered no details. The US has previously said it believed al-Qaida could have been responsible for bombing a security headquarters in Damascus in December.

Violence in Syria showed no letup on Monday, with scores of opposition fighters and regime troops again killed in fierce fighting in parts of Homs city and the surrounding province. An area north of Latakia, near the Turkish border, was targeted by helicopter gunships, which also roamed the skies of central Syria near a military base that was raided by the Free Syria Army, with the help of defectors, on Sunday.

The raid, the first large-scale assault by the Free Syria Army on a military base since the start of the Syrian uprising, has given impetus to claims that the anti-regime insurgency is gaining momentum after 16 haphazard months.

Buoyed by defectors, scores of whom are thought to have aided the attack on the al-Ghanto air defence base near Homs on Sunday, opposition fighters seized large amounts of weapons and ammunition – a rare haul during many months of battles that has seen them severely outgunned by loyalist forces.

The area targeted by helicopters near the Turkish border is home to several corridors where evidence of co-ordinated arms-smuggling into Syria has recently been confirmed. A witness to one transfer said scores of AK-47s and ammunition had been smuggled across the border and paid for in cash in the days following the Houla massacre in late May, in which at least 100 died.

Saudi Arabia and Qatar have suggested since February that they supported arming opposition groups. However, evidence of state-backed weapons runs has been difficult to find in northern Syria, where Free Syria Army units are mainly using small-arms supplied by defectors, or bought from still-serving loyalist troops.

Weapons have at times also made it across the Lebanese borders, with one supply line through the Bekaa valley delivering guns and rockets from civil war era arms bazaars and another through the far north providing more modern weaponry, some of which is believed to have come from Libya.

One of the leading opposition groups, the Syrian National Council (SNC), on Monday announced it had picked a new leader, a secular Kurd, Abdulbaset Sayda, who has lived in exile for the past 17 years. Sayda immediately urged new defections, while reaching out to minority communities of Kurds, Christians, Alawaites and Druze, many of whom have feared life after the Assad regime which has ruled the country for more than 40 years.

The SNC has been crippled by infighting since its inception more than a year ago and has had a severely strained relationship with the Free Syria Army, which has been beset by its own leadership problems.

The FSA has largely been devoid of central command and control and has operated as a series of militia franchises who each call their own shots. However, an attack on parts of the capital over the weekend appeared to show heightened co-ordination. The Syrian government claims some other FSA attacks, especially near Homs, have shown a new sense of rigour and discipline.

For now, the UN plan championed by special envoy Kofi Annan remains the centrepiece of international efforts to stop Syria from unravelling across sectarian lines. The plan has called, among other things, for both sides to agree to a ceasefire and for regime forces to pull heavy weapons back from urban centres. None of its elements have been implemented.

“The coming weeks must see an intensified and urgent international effort to stop the violence and restore hope to Syria,” said Hague. “Political transition must be based on democratic principles and reflect the needs of all Syria’s minority communities, including the Kurds, Christians and Alawites.”